Now will someone tell me why "None" imports all metadata and "All Metadata" does not? When did Kafka join the LR design team?
Quite a few people misunderstand the purpose of a Metadata Preset, and think that its purpose is to
select which of the existing data fields are to be included in the Metadata panel. However, Lightroom by default will include in the catalog all data in the image's metadata fields which already exists. Thus if, for example, a Title or GPS coordinates already exist in the image metadata, that data will automatically appear in the Metadata Panel. So the real purpose of a Metadata Preset is to provide a means where certain metadata fields can have existing data
changed (in the catalog) or where blank fields can be
populated (such as adding Copyright data, Keywords, etc.).
So using "none" during import will preserve all existing metadata, and it therefore follows that you should only create and use a Metadata Preset when you want to add or change the data in existing fields. And if you do create such a preset, you need to make sure that you only check those boxes that you are adding/changing....checking a box with the data field empty will blank out the existing metadata in that field.
Given the above, you obviously need to carefully check your "All Metadata" preset to see exactly what you are trying to do when applying it....it certainly sounds as though you haven't got it setup correctly, and if your thinking is to include all existing metadata then hopefully you now realise that you don't need any preset for that.